Modern automotive fuel tanks are subject to a permeation requirement that limits the amount of hydrocarbons which can be emitted through a fuel tank wall. To achieve permeation resistance, it has become common to form a plastic fuel tank with two or more layers. One layer serves as a permeation barrier. The most common method of forming these mutlilayer structures is to form a three layer extrusion. The extrusion has an inner and outer layer usually made from a high density polyethylene (HDPE) and a barrier layer made from ethylene-vinyl-alcohol (EVOH) therebetween. The barrier layer serves to prevent or retard the permeation of hydrocarbons through the fuel tank wall.
Various openings are formed in the fuel tank for receiving items such as the fuel delivery module, filler pipe and sensing tube. It is desirable to place these openings in an area that is punctured by a blow pin during the manufacturing process. It is not always possible to align the desired openings with the puncture. These punctures are generally closed by means of a plug. The puncture is machined into a smooth surface that receives a closure plug. Closure plugs are generally welded to the exterior surface of the fuel tank wall. Existing closure plugs provide a permeation path for hydrocarbons. Solid closure plugs made from high density polyethylene permit the passage of hydrocarbons. Additionally, the edge of the fuel tank wall may become exposed to fuel. The edge contains the interface between the barrier and the inner and outer layers. Exposure to fuel may degrade the cohesiveness of the multilayer wall.
The present invention overcomes the deficiencies of the prior art by providing a closure plug having a barrier layer. The barrier layer resists the permeation of fuel through the closure. The barrier layer of the closure plug aligns with and welds to the barrier layer of the wall. In this fashion, the invention provides a nearly uninterrupted and integral permeation resistant surface in the vicinity of the closure.